fix: Use lscpu to detect lack of confinement

[farshidtz]

Auto engine selection is expected to use the set fallback when there is no access to detect the hardware. The fallback logic is broken since the auto-connected opengl interface now grants access to read /sys/bus/pci/devices. This path was being checked to assume unconfined installation, which now produces a false positive (hardware-observe not connected, confined installation) and result in errors while reading USB devices.

$ sudo stack-utils use-engine --auto --fallback=cpu
Error: scoring engines: getting machine info: getting devices: reading sysfs usb devices: reading sys/bus/usb/devices: open sys/bus/usb/devices: permission denied

This PR changes the logic and relies on lscpu command instead, similar to how it was being done before implementing the --fallback flag. This now improved to run lscpu with the absolute system path so that staging util-linux which adds lscpu at $SNAP/usr/bin/lscpu won't get around it.

$ sudo stack-utils use-engine --auto --fallback=cpu
Hardware information is unavailable; falling back to engine "cpu".

[Copilot]

Pull request overview

This PR updates snap confinement detection used by use-engine --auto to decide whether hardware information can be collected (and thus whether --fallback should be used), replacing a sysfs-based heuristic with an lscpu-based check due to changed interface permissions.

Changes:

• Add an lscpu execution check in HardwareObservable() as a proxy for “installed without confinement”.
• Remove the previous /sys/bus/pci/devices directory-read heuristic that is no longer reliable.

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